The PHP-GTK Community Site has posted some information (and some files) to help make your editor of choice a bit more "PHP-GTK aware" when it comes to code completion.

These IDEs are able to provide code completion not only for prebuilt extension code, but also for user-level PHP code. This gives us a way to provide it to PHP-GTK developers : all it takes is a "stub" file containing PHP declarations equivalent to the PHP-GTK2 classes, interfaces, and their methods and constants.

Making it by hand would be slow (and not efficient at all) so they opted to let the Reflection API do the heavy lifting for them and generate a Gtk_Dumper output file to hand to your editor and let it parse. The file is available for download.